Chemical imaging is a new scientific discipline, which combines the chemical analysis power of optical spectroscopy, including Raman, infrared, photoluminescence and fluorescence techniques, with high-resolution optical imaging. It has powerful capability for materials characterization, process monitoring, quality control and disease-state determination. This invention relates to a system for obtaining spectroscopically resolved images of materials using electronically tunable imaging spectrometers employing liquid crystal elements.
Raman and infrared image contrast is derived from a material's intrinsic vibrational spectroscopic signature, which is highly sensitive to the composition and structure of the material and its local chemical environment. As a result, Raman and infrared imaging can be performed with little or no sample preparation and are widely applicable for materials research, failure analysis, process monitoring and clinical diagnostics.
Several approaches to Raman imaging have been demonstrated that employ means to simultaneously record spatial and Raman spectral information. Specifically, such Raman imaging data consists of 2 spatial (x,y) locations and one spectral dimension—the Raman intensity as a function of Raman frequency. Almost exclusively, modem Raman imaging methods employ multi-channel charge-coupled device (CCD) detection. CCDs are employed to record two dimensions of the three-dimensional information inherent in Raman image data sets. Raman imaging systems can be differentiated by the means they employ to collect the third dimension of information. Raman imaging systems employing dispersive monochromators coupled to CCDs have been devised that rely on two-dimensional point scanning, one-dimensional line scanning, and spatial multiplexing. In addition, Michelson interferometers have been employed in point scanning systems, while a number of tunable filter spectrometers have been described in the past several years.
Of the imaging spectrometers that have been employed for Raman imaging, including liquid crystal tunable filters (LCTFs), acousto-optic tunable filters (AOTFs) and Fabry-Perot filters, LCTFs are the most effective. In general, tunable filter methods employ wide-field laser illumination in combination with multichannel detection. The two spatial dimensions of the image are recorded directly by the CCD camera, while the multispectral information is acquired by capturing images at discrete wavelengths selected by the tunable filter. Under computer control it is possible to collect a data set with a Raman spectrum at each pixel of the image. An advantage of tunable filters is that they provide image fidelity that is limited only by the number of pixels in the camera. As a result, the use of high-definition detectors allows the efficient collection of high-definition images. Prior to the introduction of LCTFs, a key limitation of tunable filters that had handicapped Raman microscopy had been the lack of the availability of tunable filters that simultaneously provided narrow spectral bandpass, broad free spectral range and high image quality. For example, AOTF Raman imaging systems provide high throughput and broad spectral coverage, but AOTFs have distinct limitations. AOTFs suffer from broad spectral bandpass, and imaging performance is degraded appreciably from the diffraction-limited conditions. In effect, AOTFs provide spectral resolution that is an order of magnitude worse than that of a typical Raman spectrometer, and spatial resolution that is approximately 2.5 times worse than the diffraction limit.
A better alternative to the AOTF is the LCTF. In general, LCTFs are electro-optically controllable spectral bandpass filters that can function from the visible to the near-infrared. A number of LCTF designs have been demonstrated for use in multispectral imaging. LCTFs based on the Lyot filter design have been used primarily as red-green-blue (RGB) color filters and fluorescence imaging filters. A nematic LCTF based on the design of the Lyot birefringent filter has been used in a Raman imaging system. The multistage Lyot filter is comprised of a fixed retardance birefringent element and a nematic liquid crystal wave plate placed between parallel linear polarizers. The nematic liquid crystal wave plates incorporated within the Lyot filter act as electronically controlled phase retarders. The LC wave plates can be adjusted over a continuous range of retardance levels, enabling continuous tunability of wavelength. In general, Lyot filters suffer from low peak transmittance. The two main sources of optical loss in the Lyot LCTFs are absorption in the polarizers and imperfect waveplate action arising from the use of simple λ/2 plates to construct the wide-field retarder stages. An LCTF based on a Fabry-Perot design has been demonstrated for Raman microscopy. However, Fabry-Perot filters suffer from low transmittance, low out of band rejection efficiency, limited free spectral range and low spectral bandpass (25 cm−1). In addition, Fabry-Perot filters are susceptible to thermal-induced drift in spectral bandpass unless contained in temperature-controlled housings.
Raman spectroscopy has been used extensively in the prior art to characterize semiconductor materials such as the elemental semiconductors silicon and germanium, as well as compound semiconductors exemplified by III-V and II-VI semiconductors such as GaAs, GaInAs, GaInAlAs, GaAlInP, GaN, GaP, ZnSe, and CdTe. Early work on these materials used a focused or an unfocused laser beam illuminating a uniform material. Later work used a laser microprobe focused through a high quality micoscope to investigate microscopic features of interest in the materials. Images were obtained by scanning the microscopic laser spot over the surface, and recording the Raman scattered light gathered as a function of time, or by focusing the laser to a line focus and using a two dimensional detector to record one spatial dimension and one wavelength dimension. The material could then be moved in a second spatial dimension to record another scan and so on to build up a three dimensional (two spatial dimensions and one wavelength dimension) image.